1. Field of the Invention
This invention is generally directed to transducers which incorporate a vibrating diaphragm, and more specifically to planar magnetic transducers which include magnets mounted on opposite sides of a diaphragm on which an electrical conductor circuit has been applied. The invention is directed to pole elements spaced intermediate each of the magnets on opposite sides of the sound producing diaphragm which function to enhance transducer efficiency by reducing magnetic flux leakage and causing the magnetic flux field from the pole faces of the magnets adjacent the diaphragm to assume a more parallel orientation relative to the conductor circuit of the diaphragm between the magnets and the pole elements.
2. History of the Related Art
In microphone transducers, acoustic pressure variations act on a diaphragm surface causing the diaphragm to vibrate. The resultant vibrations of conductors associated with the diaphragm, while retained within a magnetic field of the transducer, create a voltage signal of similar time variance and intensity characteristics as the acoustic signal used to supply the conductors of the diaphragm. In a loudspeaker transducer, an audio signal current flows through conductors of a diaphragm. A magnetic field is created by current flowing the conductors which reacts with the magnetic field of magnets mounted in proximity to the diaphragm, thereby causing magnetic forces to act on the conductors that create sound pressure waves along the diaphragm surface which are proportional and synchronous to audio signals applied to the conductors.
Diaphragms of planar magnetic loudspeakers are normally held loose or under tension in a plane parallel to the pole faces of one or more permanent magnets so as to be in the space of the static magnetic field of the magnets. An active surface area of the diaphragm, which is an area of the diaphragm which is not constrained from motion by a rigid supporting frame to which the diaphragm is attached, is vibrated when electrical signals are provided to conductor circuits attached to the diaphragm. Conductors are attached to the diaphragm in runs which, in many transducers, are generally parallel with the edges or pole faces of the permanent magnets. The path of the conductors on the diaphragm is chosen so that current flowing therethrough produces net magnetic forces of uniform direction for all of the conductor segments or runs along the active surface of the diaphragm by causing the general direction of diaphragm motion to always be perpendicular to the diaphragm surface.
The diaphragm active surface area is chosen for particular acoustic response characteristics, such as frequency response or dispersion. The spacing of conductors and the adjacent magnetics are chosen so that the diaphragm is uniformly driven across its entire active surface area for low distortion or maximum band width. As an alternative, the conductor spacing may be chosen for optimum efficiency for a particular frequency band width or for various other reasons. The electrical circuit formed by conductor runs or segments on the diaphragm is designed concurrent with the arrangement of permanent magnets so that sufficient magnetic field strength and proper magnetic field orientation is provided to all active conductor segments or runs to achieve adequate transducer efficiency.
Conductor runs on the diaphragm may take a variety of configurations, including round or rectangular. The conductors may be bonded to a diaphragm or foil conductor patterns chemically etched from foil laminates. The conductor dimensions, compositions and circuit arrangements are often chosen to meet a desired circuit impedance requirement for maximum efficiency within practical limitations. At the present time, aluminum or aluminum-clad conductors are preferably utilized for conductors due to lower mass and lower overall mass-resistivity produced over other conductor metals. Lower mass has an inherent advantage for fast transient response and lower mass-resistivity equates to higher efficiency.
The magnet materials are chosen for cost, ease of fabrication and magnetic parameters. Optimal magnet spacing, geometry and dimensional criteria may vary the magnetic material utilized in a particular application. An "air gap dimension", the spacing between a diaphragm of magnetic transducers and the magnets thereof, should be minimized for maximum efficiency but must be chosen to allow for adequate diaphragm motion at low frequencies. The optimum spacing between adjacent magnets of each assembly is also influenced directly by the air gap length.
The advantages of planar magnetic loudspeakers over other electromagnetic arrangements is that planar magnetic loudspeakers have lower distortion and more accurate phase response when compared to cone radiator type loudspeakers. U.S. Pat. No. 3,939,312 to McKay discloses a push-pull type planar magnetic transducer arrangement wherein magnets are positioned to direct a magnetic flux across the diaphragm in a slant angle with conductor runs applied to the diaphragm. In U.S. Pat. No. 4,471,173 to Winey, another push-pull magnetic arrangement is shown wherein magnets are positioned in alternating sets of rows so that magnetic fluxes are supposed to be directed tangential to the diaphragm from the north pole face of one magnet to the south pole face of an adjacent magnet and so forth across the width of the transducer with the magnets in opposing assemblies of magnets on opposite sides of a diaphragm providing repellant magnetic forces to bound the path of the magnetic flux field.
In U.S. Pat. No. 4,337,379 to Nakaya, arrays of square magnets alternating in polarity is disclosed which are retained in two similar assemblies of equivalent magnetic pole structures with a diaphragm contoured with conductor patterns arranged to minimize resonance mode inherent in some planar transducer designs.
Each of these magnetic transducer designs and other prior art structures create a long, and therefore low, permanence path for the magnetic flux from the pole faces of the magnets proximate to the conductors carried by the sound producing diaphragms. Gauss' law dictates that the flux of each permanent magnet must form a closed loop through both poles of each magnet and take the highest permanence path from pole face to pole face. Therefore, the longer the flux path, the less efficient the transducer.